In general, the present invention relates to a technology serving as a countermeasure taken in the event of a power-supply cutoff in the course of an operation to write or erase information into or from an electrically programmable and erasable non-volatile semiconductor memory. More particularly, the present invention relates to such a technology effectively applied for example to a flash memory allowing bulk erasure of data stored therein to be carried out in predetermined units.
A flash memory employs a non-volatile storage device in each memory cell thereof. The non-volatile storage device includes a MOSFET having a dual-gate structure comprising a control gate and a floating gate. The amount of electric charge accumulated in the floating gate is varied so as to change a threshold voltage of the MOSFET in order to store information into the memory cell.
In an operation to write or erase information into or from a memory cell of a flash memory, the threshold voltage may be changed inadvertently to a low level. In this case, the threshold voltage may change in different ways depending on dispersions in memory-cell characteristics. In some cases, the threshold voltage decreases to OV or a level lower than 0 V. A state in which the threshold voltage decreases to OV or a level lower than 0 V is referred to hereafter as a deplete state. Normally, a memory cell with the threshold voltage decreased excessively is subjected to an operation known as a write-up or a write-back in order to restore the threshold voltage to a level in a predetermined range.